Due to continuous growth in the global electricity demand and the limited resource in coal, petroleum and natural gas, as well as the release of green house effect gas CO2 during their combustion, in recent years, the generation of electric energy from optical energy has become a more and more meaningful topic.
Photovoltaic cell is a device that converts directly light into electric energy. In addition to possessing two electrodes, it further includes at least one light absorption layer and an electric charge transfer layer.
Photochemical cell made of pure metallic oxide has very low photovoltaic conversion efficiency of about 0.23% under light illumination, because under sunshine, there is a very large gap (3 eV=400 mm) between valence band and conduction band. In addition, metal oxide or metal sulfide semiconductors has limited the absorption of violet and ultraviolet light. Although metal oxide or metal sulfide semiconductors are very good electric conductor, when electrolyte is illuminated under light, stable photochemical reaction will be generated and resistance will be generated in turn.
Photo-sensitive agent in photochemistry can generate photovoltaic effect under sunshine, and the photovoltaic effect can be reinforced under chemical superposition or the synergism of pigment. Photo-sensitive agent, dye or pigment can realize electronic transfer, that is, valence change, on semiconductor surface. Photovoltaic device has two functions of optical absorption and valence change. In optical absorption, electrons are carried out through pigment on surface layer, and valence electronic transfer is realized on the contact surface between semiconductor and pigment. The photovoltaic chemical system on nano metal oxide semiconductor surface has a smoother layer, which can absorb photovoltaic energy to its largest extent.
In recent years, photochemical cell develops rapidly, but in most of the cases, the art involved is very complicated, and only visible light or weak light can be absorbed, in addition, photovoltaic conversion efficiency is low, cost is high, pollution is serious, thus, mass production becomes very difficult.